Adsorbate Concentration Profile Research Articles

Analysis of the integral zeolite molecular sieve process for helium CPS application

Zeolite Molecular Sieves (ZMSs) are commonly adopted in tritium handling facilities for impurity removal from gaseous streams, particularly for tritiated water trapping. Several activities have been conducted to characterize the adsorption and desorption behaviour of the sieving materials. Instead, less attention has been put on the analysis of the integral process comprising the adsorption of the tritiated humidity, the regeneration of the molecular sieve and the recovery of the desorbed water. Focusing on the application for the EU-DEMO helium Coolant Purification System (CPS), this work presents a process simulator relying on a MATLAB dynamic model of the ZMS bed and on a RELAP5/MOD3.3 model for the regeneration loop. The dynamic model can simulate both the adsorption and regeneration modes of the ZMS by providing the temperature and the adsorbate concentration profile along the column, while the thermal-hydraulic model is used to assess the conditions of the regeneration loop. By coupling the outcomes of the two models it is possible to establish the effective regeneration efficiency of the process and the amount of the tritiated water recovered. For the case of helium CPS, the analysis demonstrates the feasibility of the proposed operative scheme and the capabilities of the regeneration procedure to ensure a 74.4 % regeneration efficiency of the ZMS bed.

Isotope microscopy visualization of the adsorption profile of 2-methylisoborneol and geosmin in powdered activated carbon.

Decreasing the particle size of powdered activated carbon may enhance its equilibrium adsorption capacity for small molecules and micropollutants, such as 2-methylisoborneol (MIB) and geosmin, as well as for macromolecules and natural organic matter. Shell adsorption, in which adsorbates do not completely penetrate the adsorbent but instead preferentially adsorb near the outer surface of the adsorbent, may explain this enhancement in equilibrium adsorption capacity. Here, we used isotope microscopy and deuterium-doped MIB and geosmin to directly visualize the solid-phase adsorbate concentration profiles of MIB and geosmin in carbon particles. The deuterium/hydrogen ratio, which we used as an index of the solid-phase concentration of MIB and geosmin, was higher in the shell region than in the inner region of carbon particles. Solid-phase concentrations of MIB and geosmin obtained from the deuterium/hydrogen ratio roughly agreed with those predicted by shell adsorption model analyses of isotherm data. The direct visualization of the localization of micropollutant adsorbates in activated carbon particles provided direct evidence of shell adsorption.

A dimensionless analysis of heat and mass transport in an adsorber with thin fins; uniform pressure approach

A numerical study on heat and mass transfer in an annular adsorbent bed assisted with radial fins for an isobaric adsorption process is performed. A uniform pressure approach is employed to determine the changes of temperature and adsorbate concentration profiles in the adsorbent bed. The governing equations which are heat transfer equation for the adsorbent bed, mass balance equation for the adsorbent particle, and conduction heat transfer equation for the thin fin are non-dimensionalized in order to reduce number of governing parameters. The number of governing parameters is reduced to four as Kutateladze number, thermal diffusivity ratio, dimensionless fin coefficient and dimensionless parameter of Γ which compares mass diffusion in the adsorbent particle to heat transfer through the adsorbent bed. Temperature and adsorbate concentration contours are plotted for different values of defined dimensionless parameters to discuss heat and mass transfer rate in the bed. The average dimensionless temperature and average adsorbate concentration throughout the adsorption process are also presented to compare heat and mass transfer rate of different cases. The values of dimensionless fin coefficient, Γ number and thermal diffusivity ratio are changed from 0.01 to 100, 1 to 10 − 5 and 0.01 to 100, respectively; while the values of Kutateladze number are 1 and 100. The obtained results revealed that heat transfer rate in an adsorbent bed can be enhanced by the fin when the values of thermal diffusivity ratio and fin coefficient are low (i.e., α ⁎ = 0.01, Λ = 0.01). Furthermore, the use of fin in an adsorbent bed with low values of Γ number (i.e. Γ = 10 − 5 ) does not increase heat transfer rate, significantly.

Direct observation of solid-phase adsorbate concentration profile in powdered activated carbon particle to elucidate mechanism of high adsorption capacity on super-powdered activated carbon

Decreasing the particle size of powdered activated carbon (PAC) by pulverization increases its adsorption capacities for natural organic matter (NOM) and polystyrene sulfonate (PSS, which is used as a model adsorbate). A shell adsorption mechanism in which NOM and PSS molecules do not completely penetrate the adsorbent particle and instead preferentially adsorb near the outer surface of the particle has been proposed as an explanation for this adsorption capacity increase. In this report, we present direct evidence to support the shell adsorption mechanism. PAC particles containing adsorbed PSS were sectioned with a focused ion beam, and the solid-phase PSS concentration profiles of the particle cross-sections were directly observed by means of field emission–scanning electron microscopy/energy-dispersive X-ray spectrometry (FE-SEM/EDXS). X-ray emission from sulfur, an index of PSS concentration, was higher in the shell region than in the inner region of the particles. The X-ray emission profile observed by EDXS did not agree completely with the solid-phase PSS concentration profile predicted by shell adsorption model analysis of the PSS isotherm data, but the observed and predicted profiles were not inconsistent when the analytical errors were considered. These EDXS results provide the first direct evidence that PSS is adsorbed mainly in the vicinity of the external surface of the PAC particles, and thus the results support the proposition that the increase in NOM and PSS adsorption capacity with decreasing particle size is due to the increase in external surface area on which the molecules can be adsorbed.

Assessment and Modeling of Adsorption Selectivities in the Transport of Mixtures of Hydrocarbons in FCC Catalysts

Adsorption experiments with up to 10 s contact time in a batch adsorber with a ternary mixture of hexane, decane, and toluene at 250 °C and a binary mixture of hexane and 1-hexene at 160 °C were used to assess equilibrium and apparent adsorption selectivities and diffusion selectivities in an equilibrium commercial FCC catalyst, and to confirm the predictions of a zeolite diffusion−adsorption model based on the Maxwell−Stefan formulation in combination with an isotherm according to the ideal adsorbed solution theory (IAST). Hexane was the reference hydrocarbon, and the decane and toluene apparent selectivities observed in the mixture were higher than those of the individual cases. The predictions of the apparent adsorption selectivities were close to the experimental values and higher than those for diluted conditions. The stronger adsorption of decane and the interactions between the various species within the zeolite particles contributed positively to decane overall transport in comparison to the other hydrocarbons in the mixture. Such interaction effects can be clearly visualized in the simulation of the transient adsorbate concentration profiles inside the zeolite particles. A similar behavior was observed in the case of the olefin in the hexane/hexene mixture.

A microscope FTIR mapping study on diffusion of hydrocarbons in single silicalite crystal particles

A simple microscope FTIR mapping method is illustrated for studying anisotropic diffusion in zeolite. This method is based on measurements of evolution of the adsorbate concentration (IR absorbance) profiles in two principal directions of a single zeolite crystal particle of a few ten microns in sizes. A mathematical model is presented and a comparison of its numerical solution with experimental data allows the determination of the diffusivity in each direction in zeolite crystals. Diffusion rates during desorption of n-hexane, n-octane and p-xylene in silicalite particles of 130 × 50 × 50 μm were experimentally studied with the microscope FTIR line mapping. Negligible changes in adsorbate concentration profiles were observed after the silicalite particle saturated with one of the three hydrocarbons was exposed to air for about 15–44 h at room temperature. Under vacuum conditions and at higher temperatures, the hexane concentration profiles developed within about 0.5 h, from which the averaged diffusivity of hexane in the a and b channels of silicalite was found 1.1×10 −10 cm 2/s (at 80°C), significantly higher than that for the c direction. Due to the intergrown microstructure of the single silicalite particles studied in this work, unusual adsorbate concentration profiles were observed in some cases. These results suggest that the adsorbate may diffuse in or out of the single crystal particle through the intercrystallite gaps connected with the straight b channels of silicalite.

Intraparticle adsorbate concentration profile for linear driving force model

The purpose of this note is to demonstrate that a very general adsorbate concentration profile within the spherical adsorbent particle can satisfy the LDF (linear driving force) model.

The chemistry of reaction-diffusion fronts investigated by microscopic LEED I–V fingerprinting

The coverage dependence of LEED I–V curves has been used to measure adsorbate concentration profiles across reaction-diffusion fronts under dynamic conditions. For reaction-diffusion fronts occurring during the catalytic oxidation of CO on Pt(100) the results are in good agreement with those from other methods and provide a successful first test of the technique. Application to the oscillating system CO/NO on Pt(100) indicates that the CO concentration oscillates out of phase and with a larger amplitude than the NO concentration.

Batch adsorption: Intraparticle adsorbate concentration profile models

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Approximation of intraparticle mass transfer in adsorption processes—II. Non-linear systems

Abstract Approximate expressions of adsorbate uptake rate by spherical pellets were derived from the solutions of the intraparticle diffusion equation by the methods of weighted residuals. For systems with linear adsorption isotherms, the classic linear driving force (LDF) expression was shown to be equivalent to the solution of the intraparticle, diffusion equation using the orthogonal collocation method with one collocation point. Alternatively, it can be shown that the LDF expression gives the best approximation (in the least-square sense) if a parabolic adsorbate concentration profile within the pellet is assumed. The use of higher-order polynomials for approximating the adsorbate concentration profile is equivalent to the use of a correspondingly larger number of collocation points in the solution of the intraparticle diffusion equation.

Prediction of activated carbon adsorption performance under high relative humidity conditions

AbstractHumidity effects on carbon adsorption were investigated using five volatile organic compounds (VOCs). Relative humidities ranged from 54% to 92%. Under these high relative humidity conditions, the Dubinin–Polanyi equation was used to analyze the results of activated carbon adsorption capacity. Based on the Dubinin–Polanyi equation and the concept of a mass transfer zone, an adsorption kinetic equation was derived to predict the carbon adsorption (MTZ) velocity and the adsorbate concentration profile in the bed.

Adsorption on stepped surfaces: The adsorbate concentration profile and its induced work-function change.

A model for the work-function change caused by adsorption on stepped surfaces is studied and discussed. The equilibrium concentration profiles of adsorbed particles at step and terrace sites at different coverage and temperature is calculated by means of a Monte Carlo simulation. The obtained profiles allow us to evaluate the work-function changes by use of the Helmholtz equation and the experimental evidence which shows that the dipole moment of the adparticles depends on the adsorption site. Our results are in agreement with available experimental data and we point out that special care must be taken in order to interpret the work-function changes measured by use of polycrystalline samples and step-``contaminated'' single crystals.